Return-Path: linux-nfs-owner@vger.kernel.org Received: from mail-ee0-f45.google.com ([74.125.83.45]:34595 "EHLO mail-ee0-f45.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933313AbaD2Jxo (ORCPT ); Tue, 29 Apr 2014 05:53:44 -0400 Message-ID: <535F76A4.4090208@gmail.com> Date: Tue, 29 Apr 2014 11:53:40 +0200 From: "Michael Kerrisk (man-pages)" MIME-Version: 1.0 To: NeilBrown CC: mtk.manpages@gmail.com, "Stefan (metze) Metzmacher" , Jeff Layton , "linux-fsdevel@vger.kernel.org" , lkml , Ganesha NFS List , Suresh Jayaraman , Trond Myklebust , Christoph Hellwig , linux-nfs , "J. Bruce Fields" Subject: Re: flock() and NFS [Was: Re: [PATCH] locks: rename file-private locks to file-description locks] References: <1398087935-14001-1-git-send-email-jlayton@redhat.com> <20140421140246.GB26358@brightrain.aerifal.cx> <535529FA.8070709@gmail.com> <20140421161004.GC26358@brightrain.aerifal.cx> <5355644C.7000801@gmail.com> <20140421184640.GD26358@brightrain.aerifal.cx> <535573E0.9080106@gmail.com> <20140421155520.3b33fbef@ipyr.poochiereds.net> <53558A73.3010602@samba.org> <5355F60C.8010004@gmail.com> <20140427145125.21e7e6c6@notabene.brown> <535CCAD2.4060304@gmail.com> <20140427200431.426c98d1@notabene.brown> <20140428072845.67f48d8e@notabene.brown> <535F6BC4.2090601@gmail.com> <20140429192458.641ebf1d@notabene.brown> In-Reply-To: <20140429192458.641ebf1d@notabene.brown> Content-Type: text/plain; charset=UTF-8 Sender: linux-nfs-owner@vger.kernel.org List-ID: On 04/29/2014 11:24 AM, NeilBrown wrote: > On Tue, 29 Apr 2014 11:07:16 +0200 "Michael Kerrisk (man-pages)" > wrote: > >> On 04/27/2014 11:28 PM, NeilBrown wrote: >>> On Sun, 27 Apr 2014 13:11:33 +0200 "Michael Kerrisk (man-pages)" >>> wrote: >>> >>>> On Sun, Apr 27, 2014 at 12:04 PM, NeilBrown wrote: >>>>> On Sun, 27 Apr 2014 11:16:02 +0200 "Michael Kerrisk (man-pages)" >>>>> wrote: >>>>> >>>>>> [Trimming some folk from CC, and adding various NFS people] >>>>>> >>>>>> On 04/27/2014 06:51 AM, NeilBrown wrote: >>>>>> >>>>>> [...] >>>>>> >>>>>>> Note to Michael: The text >>>>>>> flock() does not lock files over NFS. >>>>>>> in flock(2) is no longer accurate. The reality is ... complex. >>>>>>> See nfs(5), and search for "local_lock". >>>>>> >>>>>> Ahhh -- I see: >>>>>> http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=5eebde23223aeb0ad2d9e3be6590ff8bbfab0fc2 >>>>>> >>>>>> Thanks for the heads up. >>>>>> >>>>>> Just in general, it would be great if the flock(2) and fcntl(2) man pages >>>>>> contained correct details for NFS, of course. So, for example, if there >>>>>> are any current gotchas for NFS and fcntl() byte-range locking, I'd like >>>>>> to add those to the fcntl(2) man page. >>>>> >>>>> The only peculiarities I can think of are: >>>>> - With NFS, locking or unlocking a region forces a flush of any cached data >>>>> for that file (or maybe for the region of the file). I'm not sure if this >>>>> is worth mentioning. >>>> >>>> I agree that it's probably not necessary to mention. >>>> >>>>> - With NFSv4 the client can lose a lock if it is out of contact with the >>>>> server for a period of time. When this happens, any IO to the file by a >>>>> process which "thinks" it holds a lock will fail until that process closes >>>>> and re-opens the file. >>>>> This behaviour is since 3.12. Prior to that the client might lose and >>>>> regain the lock without ever knowing thus potentially risking corruption >>>>> (but only if client and server lost contact for an extended period). >>>> >>>> Do you have a pointer for that commit to 3.12? >>>> >>> >>> http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=ef1820f9be27b6ad158f433ab38002ab8131db4d >>> >>> did most of the work while the subsequent commit >>> >>> http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=f6de7a39c181dfb8a2c534661a53c73afb3081cd >>> >>> changed some details, added some documentation, and inverted the default >>> behaviour. >> >> Thanks for that detail. What do you think of the following text for the >> fcntl(2) man page: >> >> Before Linux 3.12, if an NFS client is out of contact with the >> server for a period of time, it might lose and regain a lock >> without ever being aware of the fact. This scenario poten‐ >> tially risks data corruption, since another process might >> acquire a lock in the intervening period and perform file I/O. >> Since Linux 3.12, if the client loses contact with the server, >> any I/O to the file by a process which "thinks" it holds a lock >> will fail until that process closes and reopens the file. A >> kernel parameter, nfs.recover_lost_locks, can be set to 1 to >> obtain the pre-3.12 behavior, whereby the client will attempt >> to recover lost locks when contact is reestablished with the >> server. Because of the attendant risk of data corruption, this >> parameter defaults to 0 (disabled). >> > > Mostly good. > > I'm just a little concerned about "if the client loses contact with the > server" in the middle there. It is no longer qualified and it isn't clear > that the "for a period of time" qualification still applied. And we should > probably quantify the period of time - which defaults to 90 seconds. > I don't remember just now the difference between > /proc/fs/nfsd/nfsv4{lease,grace}time > but this 90 seconds is one of those. > > Also this is NFSv4 specific. With NFSv3 the failure mode is the reverse. If > the server loses contact with a client then any lock stays in place > indefinitely ("why can't I read my mail"... I remember it well). > > Before Linux 3.12, if an NFSv4 client loses contact with the server > (defined as more than 90 seconds with no communication), it might lose > and regain .... Thanks, Neil. Changed as you suggest. I'd quite like to mention which of /proc/fs/nfsd/nfsv4{lease,grace}time is relevant here. I had a quick scan, but could not determine it with complete confidence. My suspicion, looking at fs/lockd/svcproc.c and fs/lockd/grace.c::locks_in_grace() is that it is /proc/fs/nfsd/nfsv4gracetime that is relevant here. Can anyone confirm? Cheers, Michael -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/